Meanwhile, in the Premier League, fourth place and a spot in next season's Champions League looks to be slipping from the Red Devils grasp, following last weekend's loss to Sunderland which saw United fall six points behind cross-town rivals City, who occupy fourth place.

With a Jose Mourinho-shaped elephant in the room, Van Gaal's Old Trafford future is in serious doubt.

But where has it all gone wrong for the United boss this season?

We take a look at the key events to have taken place since the summer...

Time Line

Louis van Gaal's demise - the timeline

Van Persie and Nani leave

After deciding against exercising the option to buy Radamel Falcao from Monaco, Louis van Gaal sanctioned the departures of two more attacking players.

Van Gaal and Robin van Persie were seemingly a match made in heaven at the 2014 World Cup, but 12 months later their relationship had soured dramatically.

Meanwhile Nani, who spent last season on loan at Sporting Lisbon, was never given a chance to prove himself to the Dutchman

Di Maria joins PSG

Angel Di Maria signed for United for a British record £59.7million fee in Van Gaal's first summer in charge and began his Old Trafford career brightly, netting three times in his first five games for the club.

However, he would go on to score just once more as he struggled to flourish in the manager's rigid setup.

The Red Devils made a £15million loss on him when he joined PSG for £44.3million.

Van Gaal spends big on imports

The quartet have all disappointed during the first half of the season.

Young French striker Anthony Martial joined for £36million in September and is the only player to have shown any sign of promise, netting seven times in 21 appearances.

The season starts well

United picked up maximum points from their opening two games of the season, recording 1-0 wins over Tottenham and Aston Villa.

A drab goalless draw against Newcastle followed but Van Gaal's men were in a strong position after three games.

Champions League qualification is sealed

Finishing fourth last campaign meant United faced the prospect of a two-legged play-off tie to reach the main draw of the competition.

A 3-1 home win coupled with a 4-0 away win gave Van Gaal's side a 7-1 aggregate victory over Club Brugge and fired them into the group stages.

First loss of the campaign

Juan Mata gave United the lead at Swansea before Andre Ayew headed the hosts level.

A Sergio Romero howler allowed Bafetimbi Gomis to grab the winner and condemn the Red Devils to their first defeat of the season.

Reports of player discontent

Senior players reportedly confronted Van Gaal to speak to him about his strict training methods at the start of September.

The Telegraph claimed players were also disappointed with his treatment of defender Rafael Da Silva, who left the club to join Lyon during the transfer window after being told he didn't have a future at Old Trafford.

De Gea returns for Liverpool win

The closure of the summer transfer window brought about David de Gea's return to the United starting line-up.

After a window which looked destined to end up with him joining Real Madrid, a deadline day paperwork cock-up allowed the Old Trafford club to keep hold of their star man for another season at least.

He returned to the team for the home clash with Liverpool, which United won 3-1 courtesy of strikes from Daley Blind and Ander Herrera, as well a wonder goal from new boy Martial.

Shaw breaks leg in Euro disaster

Luke Shaw suffered an horrific leg break in United's opening Champions League group stage match away to PSV Eindhoven and was later ruled out for at least six months.

Memphis Depay put the team in front but the Red Devils ended up losing the match 2-1, putting extra pressure on the remainder of the club's group stage campaign.

Flying high in September

Wins over Southampton and Sunderland sent United to the top of the Premier League at the close of September.

A comeback win over Wolfsburg also helped get their Champions League campaign back on track.

Thumped at Arsenal

Three goals in the opening 20 minutes condemned United to a 3-0 drubbing at the hands of Arsenal.

Van Gaal's men couldn't handle the tempo of Alexis Sanchez, Theo Walcott and Mesut Ozil and were comprehensively outplayed at the Emirates, losing their place at the top of the table.

Bore draw galore

After a 3-0 win away to Everton, United went on a run of four consecutive draws - three of which were goalless.

A 1-1 draw against CSKA Moscow, preceded stalemates against Manchester City, Middlesbrough - who United eventually lost to on penalties to get knocked out of the League Cup - and Crystal Palace.

Back on track

A drab performance at home to CSKA Moscow ended with a late winning header from Wayne Rooney to put United just one win away from qualification to the Champions League knockout stage with two games to spare.

League wins against West Brom and Watford followed to put the Red Devils within a point of leaders Leicester.

Return of the draws

United were held to a 0-0 draw against PSV Eindhoven to pile the pressure on with one Champions League group game to spare.

Van Gaal's men then earned a Premier League point at Leicester before drawing 0-0 again at home to West Ham.

Four losses on the bounce

The Red Devils lose their do-or-die Champions League clash in Wolfsburg to crash out of the competition before three consecutive top flight defeats to Bournemouth, Norwich and Stoke put Van Gaal on the brink.

Back from the brink?

The Stoke defeat is followed by a much-improved 0-0 draw against Chelsea and narrow wins over Swansea in the Premier League and Sheffield Wednesday in the FA Cup. The United faithful has something to cheer about with a 1-0 win at Liverpool.

Old Trafford turns?

A tepid display against Southampton sees United fail to score in the first half at Old Trafford for the eleventh match in a row. After mustering just one shot on goal, the Red Devils end up losing 1-0 to Saints and LVG's men are booed off, as the fans' mood appears to shift.

Red Devils lose at relegation-haunted Sunderland

Another meek display from Van Gaal's men sees United go down 2-1 at 19th-placed Sunderland. The Dutchman concedes that it will be "very difficult" for his side to qualify for the Champions League.

Wayne Rooney ruled out for six weeks

Another United injury crisis is headlined by the news that Wayne Rooney sustained a knee ligament injury in the Sunderland loss. The Red Devils' skipper is ruled out for six weeks, as 13 players are unavailable for the Europa League clash with Midtjylland.

United go down in Midtjylland

The Red Devils' Europa League campaign gets under way with a dour 2-1 loss to Danish side Midtjylland. David De Gea limps off in the warm-up, as Van Gaal blames Murphy's law for another poor display. The Dutchman also agrees with the travelling fans' four-letter assessment following the loss.